Switch from upx-nrv to upx for binary compression

Under Debian, the non-free upx-nrv package does not seem to
be available anymore. Using upx or upx-ucl works fine, once
installed via 'sudo apt-get install upx-ucl'.

Linux build instructions to follow, but tested with make
via cbp2make (using cbp2make-stl-rev147.tar.gz).